The present invention relates in general to a mobile radio receiver with reduced distortion and reduced signal fading, and more specifically, to a switched directional antenna utilizing predetermined antenna patterns aligned with the front, back, left, and right sides of a mobile vehicle.
A primary source of noise and distortion in radio receivers is from multipath interference. This is a localized effect resulting from interaction between separate signals from a transmitter which traverse different paths (e.g., via reflections) to reach a receiving antenna. Because of the superposition of several signals (i.e., echoes and/or direct waves), the signal strength of the received signal changes drastically and may fall below the noise floor. Based upon the differences in path lengths of each received wave, the multipath distortion or fading may include short-time delayed multipath interference and/or long-time delayed multipath interference signals. The multipath interference depends upon diverse geographic features and buildings. In an urban area with high buildings along both sides of a street, for example, the broadcast waves propagate along the street and become mixed with many short-time delayed signals. Along a river side, long-time delayed signals may be mixed with both direct and quasi-direct signals. In a basin, there may be several long-time delayed signals arriving from different directions. This variability has made it difficult to solve the problem of multipath distortion in mobile radio receivers.
A well known means for reducing multipath distortion is through use of space-diversity antennas in a radio receiver system. By switching between antenna signals from spaced apart antennas, specific multipath events can be avoided if the antenna spacing is enough to insure that both antennas will not experience the same multipath event at the same time. However, since space diversity radio receiver systems cannot select only a single wave, they cannot completely avoid multipath distortion. The distortion is especially serious in long-time delay multipath conditions, such as may exist at a riverside or in a basin.
Another technique that has been used to reduce multipath interference is known as antenna beam steering. These systems use an antenna array which is operated in a manner to receive broadcast waves from a single direction only. However, a complex controller must be employed in order to scan the antenna beam and align it with the strongest incoming broadcast signal. The known antenna arrays and controllers are undesirably complex and expensive, and are not practical for use in automotive radio receiver systems.
The present invention has the advantage of providing an inexpensive mobile radio receiver with greatly reduced multipath distortion while maintaining reasonable gain for the desired signal.
In one aspect of the invention, a radio receiver system is installed on a vehicle, wherein the vehicle has front, back, left and right sides. A plurality of antenna elements are mounted on the vehicle, each antenna element producing a respective radio-frequency signal. An antenna combiner is coupled to the antenna elements electronically combining the radio-frequency signals according to a selected one of four predetermined phase/amplitude combinations to generate a combined antenna signal. The four predetermined phase/amplitude combinations each provides a respective directivity pattern substantially aligned with the front, back, left, and right sides of the vehicle, respectively. A tuner generates a tuner signal in response to the combined antenna signal. A quality detector generates a detection signal in response to a comparison of a quality of the tuner signal with a predetermined quality. A pattern selector is coupled to the quality detector in the antenna combiner to change-over the antenna combiner to a different one of the predetermined phase/amplitude combinations in response to the detection signal.